Astronomers have detected a lensed quasar more than twice as wide as any
previously reported.

Studying such quasars can help astronomers learn more about dark matter and how
it was distributed in the early days of the universe. Quasars, compact but
luminous objects thought to be powered by super-massive black holes, are
billions of light-years away.

"Where you’ve got pockets of the most dark matter, that’s where clusters of
galaxies form," said Andrew Marble, a doctoral candidate in astronomy at the
University of Arizona in Tucson. "Wide lenses probe what the universe looked
like back in time and are a way of detecting dark matter."

Marble’s report on quasar QSO 2QZ J1435+0008, "The Most Widely Separated Lensed
QSO," will be given today as a poster at the American Astronomical Society’s
meeting in Atlanta. The presentation, poster number 120.10, will be in Hanover
Hall in the Hyatt Regency Atlanta, 265 Peachtree Street NE. Marble and UA
astronomer Chris Impey headed the research team, which also included Jane Rigby,
Kristoffer Eriksen, Bai Lei, Cathy Petry and Romeel DavÈ of UA and Lance Miller
of Oxford University.

Astronomers are searching for ways to learn more about dark matter, the majority
of matter in the universe. However, dark matter doesn’t emit visible light or
other forms of radiation. That makes dark matter extremely difficult to study,
because almost all astronomy is based on detecting radiation of one form or another.

One way to investigate dark matter is to study lensed quasars, quasars that have
a celestial object such as a galaxy directly between them and Earth. In that
case astronomers refer to the celestial body as a gravitational lens, because
gravity from the object bends the quasar’s light, much as the lens from a
magnifying glass bends and refocuses light. Lensed quasars are rare, Impey said,
because only 1 in 500 quasars has a celestial object perfectly lined up between
it and Earth.

An astronomer observing a lensed quasar from Earth actually sees multiple images
of the quasar. So seeing two quasars quite close together indicates the
possibility that rather than two quasars, the astronomer is seeing the light
from one quasar being bent in a way that generates duplicate images of the
quasar. The more massive the gravitational lens, the stronger the effect it has
on light and the farther apart the images of the quasar.

Marble and other researchers were working on a project that examined pairs of
quasars that appear to be located close together. Lance Miller had given the
team a list of such quasars and pointed out that one, QSO 2QZ J1435+0008, looked
as if it might be a lensed quasar, rather than an actual pair of quasars. He
thought that might be the case because both quasars appeared to have the same
redshift, a number analogous to the distance from Earth.

In March 2003, Marble and some of his colleagues used the 6.5-meter (21-foot)
diameter UA/Smithsonian MMT atop Mount Hopkins, Ariz., to look at the quasar
pairs. He said, "We didn’t go to the telescope expecting to find a gravitational
lens because nobody had ever seen such a wide one before."

However, when the numbers about QSO 2QZ J1435+0008, started coming in, the group
got pretty excited, he said. "We thought, ‘This might really be a wide lens,’"
but, he added, "You never know for sure until you go back to your office and
analyze the data."

The team also obtained images of the quasars with the 6.5-meter (21-foot)
diameter Magellan telescope at Las Campanas Observatory in Chile.

Once the data were analyzed, it was clear that the two images had the same
redshift. Other aspects of the spectra, measures of the radiation energy coming
from the quasars, were almost identical.

Impey said, "Quasars are not all created equal. Any two quasars at the same
redshift will not have the same spectra. If it’s a lens, the spectra should be
virtually identical." He added, "We believe we’ve discovered the
widest-separation lens yet. Ours is bigger than the others. It’s surprising."

The distance between the two images, which is measured in arc-seconds, tells the
researchers about the mass of the intervening celestial object, the
gravitational lens. The two images of QSO 2QZ J1435+0008 are 33 arc-seconds
apart, almost twice as far apart as the next-biggest gravitational lens known
and 20 times larger than the average gravitational lens.

Because the wide separation means the mass is greater than a galaxy, Impey said
the object is most likely a cluster of galaxies.

The image from the Magellan telescope shows a galaxy right in the proper place
to be the lens, he said, but that image is not detailed enough to show whether
there is actually a cluster present. The final confirmation will come when
Marble goes to the Magellan telescope in late January to make more measurements.

"The majority of the material in the universe is dark matter — the amount of
material that people and planets and stars are made of is dwarfed by dark
matter," Impey said. "These unusual wide lensing situations tell us important
things about dark matter and about the evolution and structure of the universe."

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